Sensitivity and Specificity of diagnostic tests

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Disease
Positive Negative
Test Positive True Positive (TP)
 False Positive (FP)
 TP + FP
Negative False Negative (FN)
 True Negative (TN)
 FN + TN
TP + FN
 FP + TN
Sensitivity:
Specificity:
Positive Predictive Value (PPV):
Negative Predictive Value (NPV):
Likelihood Ratio Positive (LR+):    
Likelihood Ratio Negative (LR-):
Sensitivity

The probability of the test finding disease among those who have the disease or the proportion of people with disease who have a positive test result.

Sensitivity = true positives / (true positives + false negatives)

Specificity

The probability of the test finding NO disease among those who do NOT have the disease or the proportion of people free of a disease who have a negative test.

Specificity = true negatives / (true negatives + false positives)

Positive Predictive Value (PPV)

The percentage of people with a positive test result who actually have the disease.

Positive predictive value = true positives / (true positives + false positives)

Negative Predictive Value (NPV)

The percentage of people with a negative test who do NOT have the disease.

Negative predictive value = true negatives / (true negatives + false negatives)

Likelihood Ratio Positive (LR+)

The odds that a positive test result would be found in a patient with, versus without, a disease.

Likelihood Ratio Positive (LR+) = Sensitivity / (1 - Specificity).

Likelihood Ratio Negative (LR-)

The odds that a negative test result would be found in a patient without, versus with, a disease.

Likelihood Ratio Negative (LR-) = (1- Sensitivity) / Specificity.

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Source code is available at biophp.org